Sudden silence while changing track, an "audio server died" notification, then
crackle and no sound while Ardour's faders still moved. Four scsynth SIGSEGVs in
four minutes (14:01:58, 14:02:55, 14:03:52, 14:04:53), each ~57s apart, until the
watchdog hit its bound: "GIVING UP: 3 restarts in 600s and scsynth will not stay
up." sclang stayed alive throughout, so the systemd unit read active (running)
the entire time — green unit, no sound, exactly the supervision trap.

WHERE IT DIED
Every coredump carries the same frame #1: JoshPVUGens.so, on the audio callback
thread (libpipewire -> libspa -> libjack -> libscsynth -> JoshPVUGens). The only
route into that library from this rig is SuperDirt's `spectral-freeze` module in
default-effects-extra.scd, i.e. Tidal's `# freeze`. Eight live tracks carry it,
including cafe_glace.tidal:44 — the track being played when it started.

WHY IT DIED
The server was starving before it fell over: `LocalBuf_allocBuffer: alloc failed`,
18x `Buffer UGen: no buffer data`, and `late` values of 6-13 SECONDS. SuperDirt
builds one spectral-freeze synth PER EVENT, each allocating FFT(LocalBuf(2048))
per channel, so a dense pattern under `# freeze` drains the RT pool even at the
configured memSize of 256 MB.

The difference that matters, measured in NRT with 120 overlapping synths and a
starved pool (both survive non-realtime, but their behaviour diverges):

    PV_Freeze     LocalBuf_allocBuffer: alloc failed        <- and then nothing
    PV_MagFreeze  PV_MagFreeze_next: alloc failed x6        <- detects it, bails

The core UGen guards its buffer. Josh's does not — it proceeds with the failed
allocation. Single-threaded NRT survives that; the realtime callback does not.

THE FIX
Re-`.add` "spectral-freeze2" after ~dirt.start, identical but for PV_MagFreeze —
the core-SC sibling JoshUGens' own help file compares itself to, with the same
trigger semantics (freeze > 0 freezes magnitudes). Slightly more static, since
PV_Freeze also advances phase between frames; in exchange it cannot take the
server down. Verified live: boot prints "[boot] spectral-freeze overridden",
scsynth stable, 96 SC->Ardour links re-made by the autoroute unit.

Pre-existing and NOT caused by this change: `SynthDef global_mi_verb2 not found`
appears from 14:02:31, in a boot that predates the edit. Filed separately.